1. Field of the Invention
This invention generally relates to controllers for electromechanical actuators and more particularly to controllers for piezoelectric elements.
2. Description of the Related Art
Piezo-electric actuators are used in for positioning elements in a wide range of applications. In optical test and measurement they are used for positioning of lenses, filters, polarizers, mirrors, radiation sources, detectors or a stage to which any of the aforementioned may be attached. In cameras piezo-electric actuators are used for focusing lenses.
Typically such actuators have opposing ends, one of which is fixed and the other of which is frictionally coupled with a driven member, e.g. lens. As voltage is applied to the piezo-electric actuator an expansion or contraction of the actuator takes place. Since one end of the piezo-electric actuator is fixed the expansion or contraction of the actuator causes a corresponding movement of the driven member to which it is frictionally connected. Above some threshold rate of expansion or contraction the force of the frictional coupling between the actuator and the driven member is insufficient to overcome the inertia of the driven member and there is slippage or in the extreme no movement of the driven member. By driving the piezo-electric actuator with waveforms with asymmetrical voltage/current profiles on the rising and falling edge it is possible to effect consistent and cumulative movement of the driven member in one of two directions throughout the reciprocations of the piezo-electric actuator.
U.S. Pats. No. 5,394,049 entitled xe2x80x9cPiezoelectric Actuator for Optical Alignment Screws Cross References to Co-Pending Applicationsxe2x80x9d issued on Feb. 28, 1995 and U.S. Pat. No. 5,410,206 entitled xe2x80x9cPiezoelectric Actuator for Optical Alignment Screwsxe2x80x9d issued on Apr. 25, 1995, U.S. Pat. No. 6,092,431 and entitled xe2x80x9cRotary type driving device employing electromechanical transducer and apparatus provided with the rotary type driving devicexe2x80x9d issued on Jul. 25, 2000 and U.S. Pat. No. 6,111,336 entitled xe2x80x9cDriving apparatus using transducerxe2x80x9d issued on Aug. 29, 2000 each disclose piezo-electric actuators which exhibit the above discussed principals. Each of these references is incorporated by reference as if fully set forth herein.
Each of these references discloses various analog drive mechanisms for delivering the asymmetrical waveforms to the piezo-electric actuator. These circuits rely on various analog components, e.g. resistors, capacitors, current sources and sinks in conjunction with appropriate transistor switches to deliver the required waveform to the piezo-electric actuator. There are several problems which such circuits exhibit. First, they are complex and may require a large form factor. Second, the waveforms they generate are not consistent over time since they are generated using the time constants associated with resistor capacitor combinations or of fixed current sources.
What is needed is a drive circuit with reduced cost and complexity which delivers repeatable waveforms with the desired characteristics to the piezo-electric actuator.
The present invention provides a method and apparatus for controlling a piezo-electric actuator coupled to a driven member. The piezo-electric actuator is responsive to waveforms with asymmetrical voltage/current profiles on the rising and falling edge to effect consistent and cumulative movement of the driven member in one of two directions throughout the reciprocations of the piezo-electric actuator. The waveforms are digitally generated from a stored set of numbers or a function for generating the set of numbers. The numbers correspond with the discrete digital values associated with the desired waveforms for moving the driven member in either of at least two directions. The controller may be used to drive more than one piezo-electric actuator. The controller may include responsiveness to a feedback of the position of the driven member to accurately position the driven member. The controller may also include the ability to update the function or values stored in memory so as to couple more efficiently with new or existing actuators. The controller exhibits a relatively smaller form factor and reduced complexity when compared with prior art analog drivers.
In an embodiment of the invention a controller for controlling at least one piezo actuator is disclosed. The piezo actuator is coupled frictionally with at least one positioning member to move the positioning member in either of two directions as determined by relative rates of expansion and contraction of the piezo actuator. The controller includes a logic for generating digitized pulses and an digital-to-analog (A/D) converter. The logic includes the capability to generate digitized pulses each with a rising edge and a falling edge. The relative absolute values of the corresponding average slopes of the rising edge and the falling edge of each of the digitized pulses corresponds with a selected direction of movement of the at least one positioning member. The A/D converter includes an input coupled to the logic and an output coupled to the at least one piezo actuator. The A/D converter converts the digitized pulses at the input to an analog waveform at the output. This effects the movement of the positioning member in the selected direction.
In an alternate embodiment of the invention a method for controlling at least one piezo actuator is disclosed. The method for controlling comprises the acts of:
generating digitized pulses each with a rising edge and a falling edge and with relative absolute values of corresponding average slopes of the rising edge and the falling edge of each of the digitized pulses corresponding with a selected direction of movement of the at least one positioning member;
converting said digitized pulses to an analog waveform; and
driving said at least one positioning member with said analog waveform to move sad at least one positioning member in the selected direction.
Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.